The Fairfax County Board of Supervisors is threatening to stall a crucial vote on funding for the Dulles rail until it sees contract documents that have up until now been tightly held by state and airports authority officials.

The threat came from Chairman Gerald Connolly at an hours-long public briefing Monday morning intended to lay out details of the planned expansion of Metro. The briefing was held only two weeks before a planned June 18 vote on whether the board will approve $400 million for the project’s first 11.6-mile phase from West Falls Church to Wiehle Avenue.